Riflescopes or telescope sights have been commercially available for many years to assist in the accurate aiming of firearms. An instrument of this type has objective and ocular (eyepiece) lenses mounted at opposite ends of an elongated tube, and a system of erecting lenses is positioned between the tube ends to provide an erect target image which is superimposed on a crosshair reticle or other style of aiming reference mark. The optical system may be arranged for either fixed or variable magnification of the target image, but magnification is not essential to the primary function of an optical sight which is to place the aiming reference mark and target image in the same focal plane so the shooter does not have to shift eye focus between the aiming mark and the target.
It is conventional to provide adjustable reticle elements to enable variation of the aiming point both vertically and horizontally (elevation and windage), and this adjustment may involve either movement of the reticle elements or an optical shifting of the target image with respect to the aiming point. After a riflescope is mounted on a rifle, the sight is zeroed by adjusting these controls until the bullet impacts exactly at the aiming point at one specific range. If the target range is thereafter varied from the zeroed range, the shooter must either hold the aiming point over or under the desired impact point, or make an adjustment in the setting of the horizontal elevation reticle to compensate for the arched trajectory of the bullet.
Reticle adjustments in some riflescopes are calibrated in minutes of angle, but several styles of modern riflescopes use elevation reticle adjustments which are calibrated directly in terms of yardage (or any convenient unit of linear measurement) to the target. Preferably, the adjustment is provided with interchangeable range scales for use with different types of ammunition, and a riflescope incorporating this feature is shown in detail in U.S. Pat. No. 3,990,155 the full disclosure of which is incorporated herein by reference.
A few hunters are successful in developing an ability to make accurate visual estimates of target range, but most shooters need the assistance of a rangefinding device to enable accurate aiming. It is recognized in the art that when the height of the target object is known, the height of the target image in the riflescope field of view is related to target range. This geometric relation enables a rangefinding capability to be built into a riflescope by providing a height reference marking (typically the vertical spacing between a pair of horizontal reticle wires) against which the height of the target image is compared.
For example, one of the horizontal reticle wires may be vertically adjustable to enable the target image to be framed between the two horizontal wires. Alternatively, the two horizontal wires may be fixed at a constant vertical spacing, and the power or magnification of the optical system varied to alter the height of the target image until it is framed between the fixed wires. Additional adjustments may be provided to enable calibration of the rangefinder for targets of different heights.
When target distance has been measured, the elevation reticle is adjusted to this range, and the aiming point should then coincide with the impact point. Alternatively, a relatively complex mechanism may be provided to couple the elevation reticle to the rangefinder so the elevation adjustment is made automatically as the range determination is made. Riflescopes of these several different styles are disclosed in U.S. Pat. Nos. 3,123,915, 3,386,330, 3,684,376 and 3,782,822 and a particularly comprehensive discussion is provided in U.S. Pat. No. 3,948,587.
Most hunters prefer a visual display of target range which is easily and quickly inspected to enable estimation of an elevation setting, and to provide a basis for deciding whether the range is within the capabilities of the firearm and hunter. For example, the aforementioned U.S. Pat. Nos. 3,386,330 and 3,684,376 disclose several different arrangements for displaying target range in the eyepiece field of view as the rangefinder adjustment is being made.
"Internal" range displays of this type are convenient in that the shooter does not have to move his head or reposition the firearm to see the target distance measured by the rangefinder. Thses displays, however, tend to clutter and reduce the target field of view, and may interfere with the hunter's ability to capture the target image quickly, and to maintain a clear view of a moving target.
The improvement of this invention relates to an optical system for providing an image within the eyepiece field of view of a scale or object which is physically external to the eyepiece viewfield. The image is radially inward (with respect to an optical axis of the riflescope) of the object which is external to the eyepiece viewfield, and the image occupies only a very small fixed position in the viewfield to avoid interference with the primary target field of view. The invention is discussed in detail below with respect to use in displaying target-range figures, but other types of information which can be similarly displayed are also described.